In recent years, super high temperature and high pressure boilers have attracted public attention in the field of power plants because of improved high heat efficiency. Since these boilers make steam of higher temperature and pressure compared with the conventional boilers, materials for tubes such as superheater tubes in these boilers must satisfy various requirements, relating to high temperature strength and corrosion resistance that are far more severe than that of conventional boilers. For these purposes, development of an alloy with excellent high temperature strength and high temperature corrosion resistance, particularly resistance to steam oxidizing, compared with the conventional 18-8 stainless steel, has been strongly desired.
It is generally known that increasing of the Cr content is effective to improve corrosion resistance of steel. However, the strength at temperatures of 600.degree. to 700.degree. C. of steel containing about 25% Cr, such as SUS STB310 Steel (according to Japanese Industrial Standards), is lower than that of the 18-8 stainless steel. Another problem is that toughness of high Cr stainless steel containing about 25% Cr is reduced by .sigma.-phase precipitation during service and corrosion resistance is not sufficient in severely corrosive environments.
A heat resistant steel containing a higher level of Cr, i.e. 20% to 20% of Cr, and exhibiting relatively high corrosion resistance is disclosed in Japanese Patent Public Disclosure 59-153858 (JPPD 59-153858). This heat resistant steel, however, does not have enough high temperature strength to be used as a single layered tube in the above-mentioned severe environments. Although this steel can be used for one of the layers in a double layered tube, i.e. clad-tube, in combination with a high strength alloy, such double layered tubes are very expensive to produce and not always reliable for use in actual plants.
There are other types of heat resistant alloys containing about 30% Cr which also contain Mo and W for increasing the strength, as disclosed in JPPD 60-100620, JPPD 61-172350, JPPD 61-276928 and JPPD 62-55352. High temperature strength of these heat resistant alloys, however, is not sufficient.